Devices and methods for biometric identification are becoming increasingly important for providing secure access to restricted areas or devices. Biometric sensing offers significant advantages over alternative methods of restricting access to sensitive areas and equipment to authorized users. Fingerprint sensing has become one of the most important biometric sensing techniques. However, widespread adoption of fingerprint sensing technology depends on the availability of low cost, reliable and consistent devices. The availability of low cost, small fingerprint sensor now makes them suitable for use in compact consumer devices, such as cell phones, lap top computers and personal digital assistants (PDAs) where security is a concern.
Two types of common electronic fingerprint devices using silicon based CMOS technology have been introduced into the market, and have largely replaced prior optical sensing devices. The first is the single-touch sensor, which requires that the user simply position his or her finger on sensor to be read. The second is the sweep-type sensor, which requires that the user move or “swipe” his or her finger across the sensor, which then reconstructs the fingerprint by capturing a series of frames comprising “images” of discrete portions of the finger being swiped. For purposes of understanding the present invention, it is not necessary to describe the details of how electronic fingerprint sensors operate. In summary, the ridges and valleys of a fingerprint are detected by an array of electrodes formed in the sensor chip. Such operation is well-known in the art, and does not constitute part of the present invention. The sweep-type fingerprint sensor offers significant advantages, insofar as it uses an array consisting of substantially fewer electrodes. This translates into smaller, more compact size and reduced manufacturing costs.
The process of fingerprint identification involves the initial step whereby the user's finger is “enrolled,” i.e., an initial image of the fingerprint is captured by the sensor (in combination with the related processing system) and stored for later comparison. Thereafter, the user will use the sensor for purposes of authentication. Both enrollment and authentication require precise and accurate capture of the fingerprint image. If the image taken from the user during enrollment has limited or unrecognizable data points, either the enrollment will be rejected or, if accepted, there will be the increased likelihood that authentication will be more difficult due to an increased rejection rate. Both of these will cause delay in obtaining authentication or inability to obtain authentication. The problems with accurate enrollment and authentication are more acute with sweep-type fingerprint sensors.